The invention relates to a process for producing directly reduced iron, liquid pig iron and steel, in which charge materials, which are formed from iron ore, preferably in lump and/or pellet form, and, if appropriate, additions, are reduced directly, in a first reduction zone, to form iron sponge, the iron sponge is smelted in a melter gasifier zone supplied with carbon carriers and oxygen-containing gas, to form liquid pig iron, and a reduction gas is generated, which gas, after off-gas cleaning, is introduced into the first reduction zone, where it is converted and drawn off as top gas, and in which process the top gas is subjected to off-gas cleaning, if appropriate is fed to a further reduction zone for direct reduction of iron ore to form iron sponge and, following reaction with the iron ore, is drawn off as export gas and is subjected to off-gas cleaning, and in which process the liquid pig iron and, if appropriate, the iron sponge from the further reduction zone are fed to a steelmaking process, in particular preferably a steelmaking process which operates on the basis of an electric furnace method, and in which process the off-gases from the steelmaking process are subjected to cleaning, and to a plant for carrying out the process.
Both during the cleaning of the reduction gas from the melter gasifier zone, of the top gas from the first reduction zone, if appropriate the cleaning of the export gas from the further reduction zone, and during the cleaning of the off-gases from the steelmaking process, dusts and/or sludges which contain both iron in oxide form and in metallic form as well as possibly coal dust are formed, depending on the cleaning method. These dusts and/or sludges constitute waste or residual materials, which it is desired to utilize in order to achieve waste-free smelting and for economic reasons in the metallurgical industry.
It is known for these residual materials to be landfilled on dumps. If possible, however, it is desired for landfilling of the residual materials and consequent loss of the products of value contained therein to be avoided and for the residual materials to be utilized with as little input of energy as possible and making optimum use of their products of value.
A process for utilizing waste and residual materials which contain iron in oxide form and/or iron in metallic form as well as carbon is known from AT-B-376,241. In this process, solids particles are separated out of the reduction gas and out of the top gas leaving the direct reduction zone in cyclones, and the solids which have been separated out are mixed with binder, such as iron oxide dust, are briquetted and are fed exclusively to the melter gasifier zone, the iron oxide dust being taken from a furnace-gas cleaning plant, so that the solids produced in the furnace-gas cleaning plant can also be utilized.
It is known from DE-A-41 23 626 for residual smelting plant materials to be agglomerated with the aid of binders, slag-forming agents and reducing agents and for the agglomerates to be introduced into the top burden area of a smelting unit, the preheating and drying of the agglomerates taking place in this burden area of the smelting unit. The burden passes through the smelting unit in counter current, initially passing into a reduction area provided in the interior of the smelting unit, after which it is smelted in the lower area of the smelting unit.
It is known from EP-A-0,623,684 for waste and residual materials containing coal dust and iron in metallic and oxide form to be collected separately in three groups according to their chemical compositions, in which case the first group is to contain primarily iron in oxide form, the second group is to contain primarily iron in metallic form, and the third group is to contain primarily carbon-containing substances. The utilization is brought about by employing the substances of the first group in the direct reduction zone and the substances belonging to the second and third groups directly in the melter gasifier zone.
This process has proven useful in particular if the waste and residual materials are formed separately, i.e. in groups, in accordance with the difference substances mentioned. However, sorting of the waste and residual materials from the metallurgical industry if iron in oxide form and iron in metallic form, as well as carbon, are produced in mixed form would be too costly.
WO 96/22950 has disclosed a process for utilizing dusts which are produced during the reduction of iron ore using a reduction gas and are separated out as sludges in a scrubber, the sludges being dewatered and used as starting material for cement production.
A common feature of all the above processes is that in each case only some of the dusts and/or sludges produced in the metallurgical industry are utilized. Other waste materials, some of which contain high concentrations of heavy metals and/or non ferrous metals, have to be disposed of in a conventional way, i.e. at landfill sites.
It is known from WO 96/34120 for iron-containing residual smelting materials which are separated out as sludges during the scrubbing of off-gases, i.e., for example, furnace gas, reduction gas and converter off-gas, to be agglomerated and used entirely in a steelmaking process which operates using the basic oxygen process, together with pig iron, scrap and/or iron ore and/or iron sponge. A drawback of this process is that high levels of accompanying elements which are undesirable for steelmaking, such as non-ferrous metals and heavy metals, are present in the steelmaking process owing to the reintroduction of the agglomerates. For this reason, a proportion of the sludges which have been separated out has to be continuously ejected. This results in high operating costs for the process, since the ejected proportion of the sludges again has to be landfilled. This process is also unsuitable for implementing a closed circuit in the metallurgical industry.
The invention aims to avoid the drawbacks which are known from the prior art and is based on the object of providing a process for producing iron sponge, liquid pig iron and steel, in which all the dusts and/or sludges which are separated out during the off-gas cleaning are fed for further utilization. In particular, it is intended for the process according to the invention to enable the dusts and/or sludges which are separated out to be fed to various utilization options irrespective of their composition, i.e. the off-gas from which they are separated. Furthermore, it is intended that landfilling, which, owing to the levels of heavy metal compounds, was hitherto required during the utilization of such dusts and/or sludges, should be avoided altogether.
According to the invention, this object is achieved by the fact that the iron-containing dusts and/or sludges which are separated off during the cleaning of the off-gases from the steelmaking process are agglomerated together with other dusts and/or sludges which are formed during the off-gas cleaning of the reduction gas from the melter gasifier zone, during the off-gas cleaning of the top gas from the first reduction zone, and if appropriate, during the off-gas cleaning of the export gas from the further reduction zone, and the agglomerates are fed for further utilization as charge materials for iron metallurgy smelting and/or reduction processes and/or cement production, and that the liquid pig iron produced and, if appropriate, the iron sponge from the further reduction zone, as well as any dusts and/or sludges produced during the process and/or agglomerates formed therefrom, as well as, if appropriate, plant scrap, form the only iron-containing charge materials for the steelmaking process.
The process according to the invention for the first time makes it possible for all the dusts and/or sludges which are separated out during the off-gas cleaning in a system comprising iron sponge production, pig iron production and steelmaking, to be agglomerated together and for the mixed agglomerate produced to be fed for further utilization. Since the only iron-containing charge materials for the steelmaking process are the product from the melter gasifier zone, i.e. liquid pig iron and, if appropriate, the product from a further reduction zone, i.e. iron sponge, the dusts and/or sludges which are separated out during the off-gas cleaning of the steelmaking process are free of heavy-metal components.
Advantageously, the top gas which leaves the first reduction zone, the possible export gas which leaves the further reduction zone, as well as at least part of the reduction gas which leaves the melter gasifier zone are subjected to gas scrubbing, and the sludges to be agglomerated which are produced in each case undergo further treatment together. This makes it possible to minimize investment costs.
According to a preferred embodiment of the process according to the invention, the sludges which are to be agglomerated are firstly dewatered down to a residual moisture content, with the result that the volume of the sludges is reduced and handling of the sludges in subsequent processing steps is facilitated. Advantageously, the off-gases which leave the steelmaking process are subjected to dry dedusting, and the resultant dusts are treated further together with the dewatered sludges which are to be agglomerated.
The dewatered sludges which are to be agglomerated are mixed, preferably in a two-step continuous process, with the dusts produced during the dry dedusting of the steelmaking process, with further oxide dusts, calcined lime and, if appropriate, coal dust, and are then granulated. The oxide dusts are expediently derived from a plant for carrying out the process, for example from the product dedusting of the further reduction zone, and/or from the casting bay dedusting unit of a steel-processing process which is connected downstream of a plant for carrying out the process.
The granules are expediently dried before being re-utilized. This increases both the strength of the granules and their thermal stability.
In previous steelmaking processes it was and remains necessary to purchase and use scrap, so-called external scrap. This external scrap contains, inter alia, heavy metals, such as lead and zinc, and is responsible for the known problems of steelmaking which result from high levels of these heavy metals being formed in the gas phase of the electric furnace. In the process according to the invention, the use of external scrap is no longer required, since liquid pig iron and optionally iron sponge form the charge materials for the steelmaking process, and because the mixed agglomerate can be used to very good effect as scrap replacement material, owing to its oxidic iron content. At the very least, so-called plant scrap, as produced during a steel-processing process connected downstream of the process according to the invention, can be used in the steelmaking process. However, this plant scrap is free of heavy metals and consequently does not cause any heavy metals to be introduced.
According to the invention, the agglomerates are advantageously re-utilized in a further steelmaking process, in particular a process which operates on the basis of the electric-furnace method or the basic oxygen process.
Furthermore, according to the invention, the re-utilization of the agglomerates is advantageously effected by feeding them to the melter gasifier zone and/or the first reduction zone. Due to their carbon content, the agglomerates contribute valuable energy to the melter gasifier zone. In the first reduction zone, the oxidic iron content is reduced to form metallic iron or iron sponge, while the carbon content of the agglomerates is partially converted into reduction gas, making a valuable contribution to the quality of the top gas.
According to a further embodiment, the re-utilization of the agglomerates is effected by feeding them to a blast-furnace process. Owing to the chemical composition of the agglomerates, and owing to the mechanical strength which can be achieved, they are eminently suitable for use in a blast-furnace process, in which case the carbon content once again contributes energy.
Advantageously, according to the invention, the agglomerates are used as starting material for cement production. Since the components of the cement material which are required for the production of cement, namely iron oxides, silicon oxide, aluminium oxide and calcium oxide or calcium hydroxide are already present in the agglomerates, these agglomerates may, for example, be added to a tubular rotary kiln installation for production of cement.
Material flow rates in the process according to the invention (with a further reduction zone present):
wet scrubber sludges (dry matter) and dusts:
The granules produced according to the invention comprise the following principal components (in percent by weight, based on dry matter):